1. Field of the Invention
The present invention relates to a method of coding a moving image, a method of decoding a moving image, a moving image coding device, a moving image decoding device, a recording medium storing a program for coding a moving image, a recording medium storing a program for decoding a moving image, and a recording medium storing coded moving image data. In particular, the present invention relates to technologies for coding and decoding, with high efficiency and facility, moving image data composed of a plurality of frame images.
2. Description of the Related Art
In recent years, network systems such as the Internet are on their way to being established as an infrastructure. Under these circumstances, motion picture and other digital deliveries, digital broadcasting, visual telephone, video conference system, and the like have been realized. Generally, in transmitting moving image data of digital form, moving images are compressed (coded) at the transmitting side for higher transmission efficiency. At the receiving side, the compressed data is extracted (decoded) into the original moving image data.
The following is a list of conventionally-used techniques for coding and decoding digital image data. The techniques (1)-(4) below are also frequently used to code still image data as well as moving image data.
(1) Entropy Coding
This is a technique of assigning codes of different lengths (variable length code) in accordance with the frequencies of occurrence of data. This includes Huffman coding, DPCM (Differential Pulse Coded Modulation) coding, and predicting coding.
(2) Conversion Coding
This is a technique in which longer codes are assigned to lower frequency components and shorter codes are assigned to higher frequency components out of the space frequency components of an image obtained through a two-dimensional Fourier transform, taking advantage of the fact that higher frequency components have smaller dispersions.
(3) Vector Quantization
This is a technique in which image signal waveforms per se are regarded as vectors, and the presence of deviations in the frequencies of occurrence of vectors due to image correlation is utilized for increasing the coding efficiency.
(4) DCT Coding
This is a technique of subjecting block images to a discrete cosine transform (DCT) to cut off high frequency components. This is employed in JPEG (Joint Photographic image coding Experts Group) and MPEG (Moving Picture coding Experts Group).
(5) Background Differential Coding
This is a technique of removing motionless background data from moving image data composed of a plurality of frame images, so as to transmit only the data of target objects.
(6) Interframe Coding
This is a technique of coding differences with respect to previous frame images out of moving image data that consists of a plurality of frame images.
(7) Motion Compensative
This is an extended technique for interframe coding, in which the motions of target objects are detected from among consecutive frames so that images shifted by the amounts of the motions are used as interframe predictable images.
(8) Recognition Coding
This is a technique of tracking, as in broadcasting a ball game where only the motions of players (target objects) change against a constant background, the feature vectors (positions, sizes, shapes, textures, or the like) of the players to code the tracks of the target objects.
In the techniques (1) to (4) above, coding is performed for each frame image; therefore, moving image data composed of a plurality of frame images is low in compression efficiency. In the techniques (5) to (8) above, the correlation among frame images arranged in a time series is utilized to improve the compression efficiency, whereas the coding requires an enormous amount of information or necessitates pre-recognition of the target object(s). Moreover, the compression rates are constant irrespective of the characteristics of the moving image data (which frequency component is contained to what extent). Accordingly, frame images containing a high proportion of high frequency components tend to deteriorate in image quality while frame images containing a high proportion of low frequency components tend to drop in compression efficiency.
The following is a list of problems in the individual techniques.
(1) Entropy Coding
Sufficient compression efficiency cannot be obtained unless the target data is widespread.
(2) Conversion Coding
There is an enormous amount of computation to be done.
(3) Vector Quantization
It is unsuitable for coding that requires real-time processing such as visual telephone because of longer coding time.
(4) DCT Coding
Since frame images are divided into blocks to reduce the amount of computation, block noise occurs in images that include rotation and other complex motions.
(5) Background Differential Coding
There is an increased amount of information when a number of target objects exist or when target objects move violently.
(6) Interframe Coding
There is an increased amount of information when a number of target objects exist or when target objects move violently.
(7) Motion Compensative
For example, MPEG involves a series of I-pictures, B-pictures, and P-pictures. I-pictures can reproduce frame images irrespective of previous and subsequent frame information. B-pictures require previous and subsequent frame information in reproducing frame images. P-pictures require previous frame information in reproducing frame images. Therefore, fast forward and fast reverse necessitate complicated processing. The coding and decoding are hard to process in parallel.
(8) Recognition Coding
Target objects must be recognized in advance.